I was looking to get a circular polarizing filter and an UV filter for my Canon 550D, 18-135mm lens. When I checked online I found these filters came in different sizes, like 62mm, 67mm, 72mm, 77mm. How do I know which is right?

Also, will there be any degradation of photo quality if I put both these filters on the lens?

Hi sfactor! No need to apologize for lack of knowledge. This is a Q/A site, so we're all hoping to learn. As for the degradation of quality issue, see this question: photo.stackexchange.com/questions/57/…
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mattdmApr 13 '11 at 12:56

@mattdm +1 for kindness and sheer coherence. Anyway, while it must be appreciated that it offers many different points of view, I think the question you linked to shows the typical bias we were talking about in meta. @sfactor you will hardly need to use both filters at the same time, and you should avoid that.
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MattiaGApr 13 '11 at 15:52

This is the thread size which means you can attach that size of filter directly. This convenient but costly. Instead, I buy my filters in the largest size (77mm usually) and have step-up rings to bridge the gap. A step-up ring costs about $12, so if you buy 77mm filters and have a 58mm, 67mm and 77mm lens, you need 2 step-up rings: 58->77 and 67->77. The only catch that you can't use a step-up ring and a lens hood at the same time. It saves lots of money considering a good polarizer costs over $200. Even if you have just two lenses with different filters you'll save. My lenses have 8 filter sizes so you can imaging how much money I saved on polarizers alone!

There will be a degradation in image quality if you use a filter.See my answer to this question. Generally, the less you pay, the more degradation there will be. UV filters are usually sold for protection but polarizers have a genuinely useful photographic purpose, attenuating glare, surface reflections and increasing color saturation of the sky and some other surfaces as a side-effect.

+1 for noting the use of step-down filters. BTW, a great polarizer doesn't have to cost $200 (although some do). In objective tests the Marumi filters tend to perform as well as the top-price ones but cost only 1/2 to 1/3 as much.
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whuberApr 13 '11 at 15:28

You can also get unwanted vignetting if you stack up multiple lenses!!! I found this out the hard way after taking around 50 photos with the UV filter and the Polarizer on. Had to go back and retake all the photos will just the Polarizer.
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John BubriskiJan 24 '12 at 18:27

@JohnBubriski: Yes, and the same thing applies to filter + step-up ring combos. However, it's generally only an issue with wide angle lenses. Roughly 24mm and wider, I'd say. With a really wide angle lens, you may have to be careful to get un-stackable filters, the sort that only let you put a single filter on the lens at a time, because the extra set of threads can cause vignetting!
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Warren YoungFeb 26 '13 at 7:12

The filter size is usually printed on the lens. If you look at the front of it, it's usually either just inside the edge before the glass (which I think is the case for your lens) or just along the outside of it. I suspect it's 67mm, but you should confirm it. It will have what looks like an O with a vertical line through it (Greek phi), followed by the size in mm. Something like Φ67mm is what you want to find.

Matt linked to an answer about UV filters in terms of effect. In general, the more glass you put in front of the image sensor, the more you impact the image quality. However, this can often be undetectable and acceptable for the gains that a particular filter may provide in other ways. For example, a polarizer may allow you to get a shot that you might not otherwise, so if there is a loss of image quality, you'd accept it. Having said all that, it's not really an issue for the most part. The only thing to be aware of is that polarizer will reduce the light by a couple of stops.

It looks like it... Close enough for government work. :)
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John Cavan♦Apr 13 '11 at 13:06

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good answer. To be picky: "...reduce the light by a couple of stops" citation needed. In past years I often used a cokin polarizer on a Panasonic Lumix FZ8 and I found out the meter never showed any difference. I mention this to say that's the only context I used a polarizer in, it could be different with another (better) camera/lens. The FZ8's sensor sucks compared to newer ones, but the camera as a whole is an excellent compact. This leads me to believe it didn't get fooled by the filter and a polarizer actually didn't affect how much light came in.
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MattiaGApr 13 '11 at 14:49

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@Mattia - The job of the polarizer is to absorb certain polarized light so it can't but reduce the amount of light coming through.
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John Cavan♦Apr 13 '11 at 14:53

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@Itai nah, only ISO 100 had decent quality on that camera. I looked for examples because I couldn't believe the difference was so big and I found out two silly yet appropriate shots this one and this one. They actually differ by one stop, which is a bit in favor of and a bit more against my statement :-)
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MattiaGApr 13 '11 at 15:29

Your question about diameter has already been answered I see. As for the stacking question, stacking filters should be avoided unless absolutely necessary. Each filter adds two new surfaces that can be dirty and/or scratched, which is a bad thing. Each filter also adds two new surfaces which can cause flare and reflections, which is a worse thing.

As a general principle, ditch the UV filter. It serves absolutely no photographic purpose on a digital camera. It gives some small measure of protection for the front element, but so does a lens-hood which in addition does serve a photographic purpose by reducing flare. Add a filter only when you need to - the only ones you need to, largely speaking, are polarizing filters and neutral gray filters.

No matter what filter size you use, you always want to put the polarizing filter on last. The end of the polarizing filter rotates so that you can filter the light from any direction (or camera orientation), so you don't want to have anything on the end of that.

I'm not even sure if my polarizing filter (B+W) has threads to accept additional filters stacked on top now that I think about it.